Fix adjoining transparent face rendering

- Render both faces when two different transparent faces are next to
  each other.

polymer/math.h

@@ -521,6 +521,16 @@ struct Vector4f {
   }
 };
 
+struct BoundingBox {
+  Vector3f min;
+  Vector3f max;
+
+  inline bool Intersects(const BoundingBox& b) const {
+    return (min.x <= b.max.x && max.x >= b.min.x && min.y <= b.max.y && max.y >= b.min.y && min.z <= b.max.z &&
+            max.z >= b.min.z);
+  }
+};
+
 // Column major matrix4x4
 struct mat4 {
   float data[4][4];

polymer/render/block_mesher.cpp

@@ -194,12 +194,8 @@ FaceQuad GetFaceQuad(BlockElement& element, BlockFace direction) {
 inline bool IsOccluding(BlockModel* from, BlockModel* to, BlockFace face) {
   BlockFace opposite_face = world::GetOppositeFace(face);
 
-  bool from_is_transparent = !HasOccludableFace(*from, face);
-  bool to_is_transparent = !HasOccludableFace(*to, opposite_face);
-
   // TODO: Clean this up once rotation is settled.
   if (to->element_count == 0) return false;
-  //if (from->has_variant_rotation || to->has_variant_rotation) return false;
   if (to->has_leaves || !to->has_shaded) return false;
 
   for (size_t i = 0; i < from->element_count; ++i) {
@@ -220,16 +216,16 @@ inline bool IsOccluding(BlockModel* from, BlockModel* to, BlockFace face) {
       // Check if the element of the 'to' model fully occludes the 'from' face
       if (to_start.x <= from_start.x && to_start.y <= from_start.y && to_start.z <= from_start.z &&
           to_end.x >= from_end.x && to_end.y >= from_end.y && to_end.z >= from_end.z) {
-        if (to_is_transparent) {
-          if (from_is_transparent) {
-            return true;
+        if (to_face.transparency) {
+          if (from_face.transparency) {
+            return from_face.texture_id == to_face.texture_id;
           }
           return false;
         }
 
-        if (from_is_transparent) {
-          if (to_is_transparent) {
-            return true;
+        if (from_face.transparency) {
+          if (to_face.transparency) {
+            return from_face.texture_id == to_face.texture_id;
           }
           return false;
         }

polymer/world/block.h

@@ -48,6 +48,31 @@ struct FaceQuad {
   Vector2f br_uv;
   Vector2f tl_uv;
   Vector2f tr_uv;
+
+  inline BoundingBox GetBoundsAt(const Vector3f& v) {
+    BoundingBox result;
+
+    result.min.x = ChooseMin(bl_pos.x, br_pos.x, tl_pos.x, tr_pos.x) + v.x;
+    result.min.y = ChooseMin(bl_pos.y, br_pos.y, tl_pos.y, tr_pos.y) + v.y;
+
+    result.max.x = ChooseMax(bl_pos.x, br_pos.x, tl_pos.x, tr_pos.x) + v.x;
+    result.max.y = ChooseMax(bl_pos.y, br_pos.y, tl_pos.y, tr_pos.y) + v.y;
+
+    return result;
+  }
+
+private:
+  inline static float ChooseMin(float x0, float x1, float x2, float x3) {
+    float m1 = x0 < x1 ? x0 : x1;
+    float m2 = x2 < x3 ? x2 : x3;
+    return m1 < m2 ? m1 : m2;
+  }
+
+  inline static float ChooseMax(float x0, float x1, float x2, float x3) {
+    float m1 = x0 > x1 ? x0 : x1;
+    float m2 = x2 > x3 ? x2 : x3;
+    return m1 > m2 ? m1 : m2;
+  }
 };
 
 struct RenderableFace {